MECHANICAL SEPARATION •
reached. If only 200 Ib. capacity can be obtained then there is undoubtedly tremendous difficulties in settling and thickening the material for the nitration work and it has approached in state of fine subdivision the ceramic clays for the preparation of which the thickening devices which have already been described and vacuum filters are but little suited.
Percentage of Solid Passing to Vacuum Filters.—This should be where possible not less than in the proportion of 1:1. The thicker the material to be filtered flows to the filter the greater its capacity. This is shown by the results obtained by grinding a copper ore to pass an 80-mesh screen. This yielded a granular material which settled and filtered readily. The tests were made on a filter leaf 11 by 13 in. and the cake was allowed in each test to load up for a period of 3 minutes and vacuum was maintained for an additional 2 min. to dry the cakes formed.
number Liquid, per cent Vacuum, inches Wet weight, ounces Dry weight, ounces Thickness, inches Moisture, per cent
1 75 15 6 4K M 25.0
2 50 16 44 33K H 23.8
3 39 17 78 59 M 24.3
The exigencies of a process may however require a large proportion of liquid. Thus in cold saccharate work the molasses is diluted with water before the introduction of lime and the material filtered only has about 10 per cent solids in it. The calcium saccharate cake made has about 65 per cent moisture in it, which is not objectionable since carbonating the cake to release the sugar follows the filtering operation.
Thickness of Cake and Moisture Content.—Where there are irregular conditions the thickness of the cake maybe as little as >g in. The ordinary cake thickness ranges from K to M. in. A cake of greater thickness can be made if it is built up of extra porous material. The more finely divided the solids are and the more colloidal they are the thinner the cake must and will be and unfortunately the greater will be its moisture content.
The moisture content is commonly understated. Crystalline solids will often make a cake under 10 per cent moisture but amorphous solids will make a cake running from 10 to 20 per cent moisture and up to as high as 30 per cent or more. On colloidal material the moisture may run up to 30 per cent and not show free moisture after being scraped from the filter and piled. The range of moisture in Western flotation concentrates is from 10 to 20 per cent, occasionally higher. With this substance yielding a high percentage of water in the cake the aim is to reduce the moisture only to the point where water will not separate out in the railroad cars. Drying of material of this kind is so difficult and costly that it is not frequently attempted.1
Un a recent appearing device for drying this sort of material the slime is forced through a large plate pierced with humorous holes resembling a similar plate used in meat grinders. The forcing of the slime through the holes is secured by the same means as is employed in meat grinders, a worm. The slime is formed through the pierced plate like a great mass of spaghetti and issued at the same rate of travel its the receiving steel plate conveyor placed below the pierced plate. The conveyor passes through a drying oven. The object of course of this invention is to promote drying by creating a large amount of drying surface.